The present invention relates generally to the field of managing overload conditions in telecommunication switching equipment, and specifically to the field of controlling overloads in an integrated MSC/HLR switch used in GSM systems.
A GSM (Global System for Mobile communications) cellular radio system includes several Base Station Subsystems (BSSs) interconnected by one or more Mobile Switching Centers (MSCs). Each BSS provides radio links to Mobile Subsystems (MSs) in a respective cell, and at least one of the MSCs connects to the Public Switched Telephone Network (PSTN).
The MSCs control call routing. For example, the MSCs route calls from the PSTN to the BSSs currently serving called MSs, and from BSSs serving calling MSs to the PSTN for connecting to landline telephones. MSCs also route calls between BSSs for inter-MS calls.
In order to route calls to the BSSs currently serving called MSs, MSCs need to identify the cell in which each MS is currently located. To do so, the MSCs access a Home Location Register (HLR), which is a database storing the current location of all mobile stations in a GSM Public Land Network. The HLR also stores information about the services enabled for each MS and information used to control access by the MSs to the cellular radio system.
To maintain and provide current information, the HLR must process authentication requests, updates to MS location information, and updates to subscriber service information. In addition, The HLR must also download information to a Visitor Location Register (VLR) associated with each MSC. The VLR temporarily stores subscriber data associated with MSs currently located in cells served by the MSC. The HLR downloads subscriber data to the VLR in response to requests from the VLR to the HLR.
In a typical large GSM system, different systems implement the MSC and HLR functions, and the overload controls for both systems also differ. For example, in such systems, a CPU along with several peripheral processors provide each MSC function. The peripheral processors connect to BSSs, other MSCs, or switches of the PSTN. The interface between the peripheral processors and the BSSs is called the A-interface.
One technique for responding to MSC overload conditions is described in a copending patent application, U.S. Ser. No. 08/319,678 to Gao, et al., filed on Oct. 7, 1994 and entitled "Overload Control for a Central Processor in the Switching Network of a Mobile Communications Systems," which is incorporated herein by reference. To keep the MSC CPU operating efficiently, the MSC peripherals in this technique monitor the load on the MSC CPU and refuse transactions on the A-interfaces when the CPU is overloaded. The peripherals determine the possible overload condition from the queuing delay for transactions awaiting processing that the MSC CPU reports. When the queuing delay exceeds a threshold, the peripherals stop sending CPU transactions relating to the initiation of new calls. Calls for which processing has been started can complete, but new calls cannot initiate until the overload condition alleviates.
This overload control is particularly effective because the peripherals throttle the incoming transactions before they reach the CPU, thus relieving the CPU from devoting processing power to this task. The MSC peripherals can distinguish different types of transactions because the protocol used on the A-interface is "connection-oriented," meaning the transactions are read by each processor on the transmission path.
In contrast, typical large GSM systems implement the Home Location Register (HLR) function using a server connected to the MSCs via C-interfaces and D-interfaces. The HLR server monitors the number of incoming transactions in a queue awaiting processing. If the number of incoming transactions in the queue exceeds a "major overload" threshold, the HLR server discards incoming transactions and so informs the source of those transactions. This allows the source of the transactions to resend the transactions later when the overload condition may have been cleared. If the number of incoming transactions in the queue exceeds a "critical overload" threshold, the HLR server aborts the incoming transactions without even informing the source of the transactions.
Even if a system implements the HLR function with a CPU connected to peripherals, overload control for the HLR function will still differ from that for MSC functions because the peripherals on the C-interfaces and D-interface cannot throttle the incoming transactions. Those peripherals use a "connectionless," TCAP (Transaction Capabilities Address Point) protocol under which each processor on the transmission path reads only the address portion of the transaction. Those peripherals thus cannot distinguish different transaction types.
Smaller GSM systems present a different problem. In such systems, it is more cost effective to provide the MSC and HLR functions with a single CPU. This design requires controlling overloads to keep the single CPU operating efficiently while recognizing the relative priorities of the MSC and HLR roles. On the other hand, such an integrated design offers unique advantages.
In light of the foregoing, there is a need for overload control for an integrated switch providing both MSC and HLR functions with a single CPU.
Another need is for such control while recognizing the proper priorities of the MSC and HLR operations.
Still another need is for overload control that follows accepted protocols.
Additional features and advantages of the invention will be set forth in the following description and will be apparent from the description and practice of the invention.